An omicron-specific Covid vaccine will be ready by March but some experts warn it could be "too late" due to the variant's highly transmissible nature.

On Monday, Pfizer CEO Albert Bourla told CNBC that its vaccine with BioNTech that targets omicron — and other variants that are currently circulating — will be ready for distribution by spring and that the company has already started manufacturing doses.

But an omicron-targeted vaccine was needed in December, says Dr. William Moss, executive director of the International Vaccine Access Center at the Johns Hopkins Bloomberg School of Public Health. "It still could be valuable but I do think in many ways, it's too late" for the current omicron wave, Moss says.

Dr. Shaun Truelove, an infectious disease epidemiologist at Johns Hopkins Bloomberg School of Public Health, agrees: "Given how quickly this [variant] is happening, [the targeted vaccine] may not matter because everybody's going to be infected," says Truelove, a member of The Covid Scenario Modeling Hub, a team of researchers who make Covid projections.

The country has been seeing record Covid infections: More than 95% of reported Covid cases were due to the omicron variant as of early January, according to the CDC. On Sunday, over 307,000 new cases of Covid were reported, according to Johns Hopkins.

If an omicron-targeted vaccine had been available earlier, it "might have been sufficient to prevent some of these illnesses and better protect our workforce, particularly health care workers," says Moss. "The assumption is that an omicron-specific booster would have high vaccine effectiveness against infection, at least temporarily, but this is not known."

But the variant spread so quickly that vaccine developers could not make a targeted vaccine in time.

Pfizer CEO Bourla also said it is still not clear whether or not the new vaccine is needed or how it could be used.

But given both Covid's and omicron's unpredictability, and with new variants likely to emerge, having a vaccine that targets omicron and other variants could be useful in some way, at some point, experts say. "In short, I think there will be some value to those who remained uninfected, assuming omicron continues to be the dominant variant, but the impact of an omicron-specific vaccine will be much less than if it were available earlier before the [surge] in infections," Moss says.

Truelove agrees, and adds that we don't know long long omicron infection-induced immunity will last, and an omicron-specific booster could provide "substantial benefit" if immunity wanes, potentially even against future variants.

"But it's not really possible to know what those impacts will be as of now," he says.

Moderna CEO Stephane Bancel told CNBC on Monday that it is also working on an omicron-specific booster, which will likely enter clinical trials soon.

Bancel also said on Thursday that a fourth Covid shot may be needed in the fall as the efficacy of boosters will likely decline over time.

Dr. Mark Sawyer, an infectious disease specialist at Rady Children's Hospital, who served on the FDA advisory committee that approved Covid vaccines in 2020, says while it's too early to predict if or when a fourth vaccine shot is needed, the fall is plausible.

"[R]espiratory illness is more common in the winter, so boosting people in the fall makes sense," Sawyer says.

Israel has already given out more than 250,000 fourth doses since early January to Israelis aged 60 and over, medical workers and those who are immunosuppressed, according to The Times of Israel. Early data from Israel shows that a fourth dose does increase antibody levels, says Dr. David Hirschwerk, infectious disease specialist and medical director at Northwell Health's North Shore University Hospital.

And fourth doses of Covid vaccines for some individuals in the U.S. who are moderately to severely immunocompromised could roll out starting this week, according to The New York Times. The CDC approved a fourth dose for that group in October.

But to what extent that will be recommended to the general public down the road, it's still too early to say. "I still think we need to have a more complete understanding of what our impact has been from people receiving a third dose," Hirschwerk says.

One thing that has become clear is that two doses aren't enough protection against omicron. "There is good evidence that the third dose is providing a lot more protection against omicron," Truelove says. Bourla also said Monday that two shots do not provide robust protection against infection with that variant.

As of Monday, roughly 63% of the U.S. population is fully vaccinated, according to the Centers for Disease Control and Prevention. Only 36% of those people have received a booster dose.

This story has been updated to include additional comments from Bourla, Moss and Truelove.

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This story has been updated to include Bourla's comments that two vaccine shots are not sufficient against omicron.